1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having copper wirings, and particularly to a method of forming an upper wiring on a copper wiring with satisfactory adhesion.
2. Description of the Related Art
With micro-fabrication of a semiconductor element or chip, an RC-delay share of a signal delay of the element is becoming innegligible. Therefore, the use of Cu (1.8 μΩ·cm) smaller in specific resistivity than an aluminum (Al) alloy (3 μΩ·cm) has been discussed from the viewpoint of a wiring material. A self-diffusion coefficient (3.04×10−10 cm2/s) of copper (Cu) is larger than a self-diffusion coefficient (8.21×10−7 cm2/s) of Al, and electromigration (EM) resistance of a Cu wiring higher than that of an AL wiring is obtained. There has however been a demand for further improvements in EM resistance of the Cu wiring. Further, the elucidation of an EM mechanism has been brought forward.
In a Cu damascene wiring technique, a silicon nitride (SiN) film or the like formed by a plasma chemical vapor deposition (CVD) method is deposited on a Cu wiring layer. It has been reported that since a Cu/SiN interface is poor in adhesion as compared with a Cu/barrier metal interface, the diffusion of Cu atoms along such an interface principally exerts control over the life of wiring. While a method of reducing an oxidation layer of a Cu surface, and the like have been discussed to improve the adhesion of an Cu/SiN interface, it is difficult to significantly improve the adhesion under the present situation.
It has also been reported that CoWP having a thickness ranging from 10 nm to 20 nm as an alternative to SiN caps over a Cu layer to thereby significantly improve EM resistance. It can be said that the replacement of all interfaces between Cu and different types of materials with those between Cu and metals is highly significant. However, even if the interfaces between Cu and the different types of materials are changed into those with the metals, the adhesion of the interfaces depends on materials to be applied.
A description will be made of the relationship between Cu and barrier metals by way of example. It has been reported that a Cu/Ta structure is improved in EM resistance as compared with a Cu/TaN structure. Cu thin films deposited on the barrier metals of their references are heat-treated at a high temperature and thereafter compared as to surface mophology. Consequently, the Cu film is coagulated in the case of the Cu/TaN structure. On the other hand, the Cu/Ta structure holds a smooth continuous film. The ability of the Cu/Ta structure to maintain the smooth surface of Cu film means that the adhesion of the interface is satisfactory. It is apparent that the barrier metal good in adhesion is improved in EM resistance as compared with Cu. This idea is estimated to be similar even as to a cap metal on Cu.
In addition to the application of the barrier metal good in adhesion to Cu, a method of forming an alloy layer of Cu between a Cu wiring and a barrier metal to thereby improve adhesion of Cu to the barrier metal has been disclosed in, for example, prior art documents (see Japanese Patent Application Nos. Hei 7(1995)-235538 and 8(1996)-17824 and Japanese Patent Application Nos. 2000-311897 and 2001-93976).
However, the prior art has a problem in that since a Cu wiring layer and a reactive layer are caused to directly react with each other to thereby form an alloy layer of copper, control on the reaction is difficult and the thickness of the alloy layer cannot be controlled. As a result, a problem arises in that wiring resistance rises.